Previous light and electron microscopic investigations, including those done by us, have revealed the general distribution patterns of nerves as well as their specific relationships with various structural components of the respiratory system. However, very little work has dealt with the development of the pulmonary innervation. In this project, the distribution of the nervous tissue in the respiratory system of the fetal and neonatal mice and human will be investigated. The electron microscope will be used as a major tool in conjunction with other histological and chemical techniques including histochemistry, fluorescent microscopy and surfactant analyses. The trachea, bronchi, lungs, as well as the pulmonary arteries and veins from mouse fetuses 10, 15, 17, 19, and 21 days old, and from postnatal mice, 2, 5, 8, 12, 18 and 21 days old will be studied. The tissues of the human respiratory system will be obtained from autopsied fetuses and infants. Efforts will be made to correlate electron microscopic observations with light microscopy and surfactant assay. The locations of the sensory and motor axons and their fine relationships with various components of the developing respiratory system will be investigated. Particular attention will be paid to follow the development of the sensory receptors demonstrated physiologically, and the adrenergic and cholinergic endings. The results of this research will contribute to the understanding in the mechanism of respiration as well as the functions of the lungs. ory